This invention relates to compression hubs for fusion reactor systems and in particular to a structure to facilitate cooling of the hub.
Superconducting magnets are used to attain intense magnetic fields in magnetic confinement systems for fusion reactors. The magnets are located in a polygonal relationship and intense forces are generated tending to draw these magnets together. Accordingly, a hub must be supplied to buck these horizontal forces.
The superconductor of the magnet which is surrounding the hub must be cooled below a critical temperature in order to obtain desired magnetic field. These critical temperatures are very close to absolute zero. For instance, the critical temperature of niobium-titanium is 9.4.degree. kelvin while niobium-tin is 18.0.degree. kelvin.
These conductors are normally operated at 4.2.degree. to 4.9.degree. kelvin so as to avoid losing the superconductivity property with slight heating, and to maintain it in intense magnetic fields. In order to obtain this low temperature, the conductors are cooled by liquid helium which boils at 4.2.degree. kelvin at atmospheric pressure. Since the temperature level is not only extremely low, but the transition from superconductivity is also very sharp, it is important that the surrounding structural material also be maintained at the extremely low temperature. This includes the hub since any heat transferred from the hub to the magnet would destroy the effectiveness of the equipment.
Prior to starting operation of the reactor, the hub is obviously at room temperature. It must be cooled about 300.degree. kelvin in order to reach the operating temperature. The practice of depending on the cooling of the magnets to remove heat conducted from the hub would require a start-up time of many weeks before operating conditions could be reached. Passing liquid helium through the hub would provide means for cooling the hub independently of the magnet cooling system. Since the coolant itself is only about 4.2.degree. kelvin, and the temperature level is very critical, it is clear that the last portion of the cooling is most critical. Very little temperature gradient exists for the purpose of cooling the hub. It follows that in order to obtain reasonable cooling time, the coolant flow path must be arranged in such a manner that no portion of the hub is a significant distance (say 25 centimeters) away from the coolant itself. The drilling of vertical openings through the hub on such spacings is unacceptable since it weakens the hub to too great an extent. The openings through the hub would increase the structural elasticity of the hub. This in turn permits the superconducting magnets to move or flex during operation. As a result of this movement, the friction will heat the conductor above the critical temperature. Consequently, such loss of stiffness in the hub cannot be tolerated.